Steam power generating facilities and steam heating plants convert the chemical energy of fossil fuels into useful thermal energy. The thermal energy released by combustion of the fuel is absorbed by water to produce steam that in turn is used to power a turbine-generator to produce electric power.
A majority of power plants burn pulverized coal as a readily available source of fossil fuel. FIG. 1 is a diagram of a typical direct-firing system for pulverized coal. A bunker 10 stores raw coal which is fed to a pulverizer 12 where the coal is ground into a fine powder having coal particles of less than 300 microns in diameter. The pulverized coal is transported to one or more burners 16 by heated air, known as "primary air" which comprises approximately 20% of the air required for combustion.
A single boiler 18 may be equipped with one or more pulverizers 12; each may serve one or more burners 16. Load control is accomplished by varying the rate of coal fed to the pulverizer(s). To accommodate large load reductions, burners and/or pulverizers are removed from service.
Coal burners 16 supply air and fuel to the furnace to maintain (1) stable combustion, (2) effective control of flame shape, and (3) thorough mixing of the fuel and air. Pulverized coal enters the furnace through a burner nozzle where the coal is mixed with additional, or "secondary" air, admitted by burner registers surrounding coal burners 16. Burner pipes and nozzles are sized so that primary air velocities are sufficiently high to maintain the coal entrained in the air flow, but sufficiently low to prevent excessive wear on the equipment by the high abrasive coal particles.
Two major types of coal burners are used in corresponding furnaces structures. In an opposed firing furnace, horizontal wall burners face the furnace from opposite walls. In a tangential firing structure, horizontal burners are tangent to a circle centered in the furnace.
Most opposed firing burners are cylindrical with a central nozzle through which coal is admitted to the furnace. Air register doors located around the periphery of the burner admit secondary air into the furnace. Coal and air are mixed and forced into the furnace for combustion. Turbulence required for good combustion is largely controlled by adjustments made to the burner itself. Flame shape can be adjusted somewhat by inserting/retracting a central impeller and by adjusting the opening of the air registers.
In a tangential firing furnace, the burners are rectangular in shape. Coal and primary air enter the furnace through fuel nozzles and secondary air enters through auxiliary nozzles located above and below the fuel nozzle. The coal/air mixture is forced into the furnace tangent to a circle about the center of the furnace. The mixture travels along a spiral trajectory toward the top of the furnace; the fireball is fed from burners located in the corners of the furnace. This spiral trajectory of the mixture provides the turbulence required for good combustion. In a tangential firing furnace, flame shape is not adjustable although air distribution can be controlled by adjusting the openings of the individual auxiliary air nozzle registers.
The transport and distribution of pulverized coal present several problems, in part because the coal, which is a solid, is transported by air (a compressible fluid). Since the densities of the two substances are different, the mixture tends not to remain homogeneous as it passes through bends in the supply piping system of the burner. Instead, the coal tends to flow toward the outside of the bend, resulting in an uneven distribution of coal over the cross-sectional area of the nozzle. Poor distribution results in an uneven wear of burner components and in reduced combustion efficiency due to poor mixing of the coal and air.
Coal distribution or flow cones have been used in burner nozzles and piping in an attempt to provide even coal mass/velocity distribution at the nozzle outlet. Uniform distribution eliminates localized areas of high velocity fluid movement which accelerates burner component wear rate. Good distribution of the pulverized coal and air mixture further improves combustion by providing an improved mix of the fuel and air in the ignition zone of the furnace. The distribution cones do this by forcing the coal to the center of the coal nozzle and allowing the coal to diverge from the center of the nozzle to completely fill the nozzle at its outlet.
Prior art distribution cones are sometimes included in opposed firing type burners by commercial manufacturers such as Babcock and Wilcox. Other manufacturers, such as Flame Refractories, Inc., provide flow cones for retrofit into existing burner equipment. In both original equipment and retrofited devices, the cone is supported by three legs radiating from, and attached to, the regulating or impeller rod and extending from the cone.
Because the prior art flow cones are attached to, and supported by, the central regulating rod, the cone tends to impede movement of the rod during reciprocal motion of the coal impeller. The cones frequently contact the inside surface of the nozzle due to warping of the cone, rod, or nozzle caused by high furnace temperatures, necessitating tedious alignment and trimming of each cone for proper operation.
Prior art designs further suffer from coal leaks between the outer edge of the cone and the inside of the coal nozzle resulting in accelerated wear of the nozzle. Leakage can result in reduction of nozzle life from a nominal 10-12 years to less than one year. Because planned outages are typically scheduled at 2 year intervals, additional maintenance time and expense are imposed.
Finally, prior art flow cones must be located near the center of the nozzle when the cones are in operation to facilitate insertion and retraction of the impeller and rod. Thus, the flow path of coal is insufficient in length downstream of the flow cone to enable the flow to fully expand or develop before coal reaches the impeller. The result is poor coal distribution; furthermore the center portion of the impeller tends to wear at a rapid rate.
A need therefore exists for a pulverized fuel distribution cone which does not impede impeller rod or impeller operation.
A need further exists for a pulverized fuel distribution cone which does not require extensive and repetitive alignment and trimming procedures for proper operation.
A need still further exists for a pulverized fuel flow cone which does not allow leakage to occur around the outer edges of the cone.
A need further exists for a pulverized fuel flow cone which is locatable an optimum distance from the impeller end of the nozzle.
A need still further exists for a pulverized fuel flow cone which is easily installed and removed.
Accordingly, an object of the invention is to provide a pulverized fuel flow cone which does not impede movement of a burner regulating/impeller rod operation of a burner impeller.
A further object of the invention is to provide a pulverized fuel cone which does not require alignment and trimming and is not susceptible to misalignment due to furnace heat and component warping.
Another object of the invention is to provide a pulverized fuel flow cone which is not subject to leakage around a peripheral portion thereof.
Still another object of the invention is to provide a pulverized fuel flow cone which is locatable an optimum distance upstream of the impeller end of a burner.
A further object of the invention is to provide a pulverized fuel flow cone which is easily installed and removed.
Still other objects and advantages of the present invention will become readily apparent to those skilled in this art from the following detailed description, wherein only the preferred embodiment of the invention are shown and described, simply by way of illustration of the best mode contemplated of carrying out the invention. As will be realized, the invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the invention. Accordingly, the drawing and description are to be regarded as illustrative in nature, and not as restrictive.